Electron beam evaporation sources have been used as a convenient means in recent years for evaporating high vapor temperature materials such as uranium and other refractive metal elements. Such a technique has been found to be useful for the evaporation of uranium in high quantities for production level isotope separation, as is discussed in commonly assigned U.S. Pat. Nos. 3,772,519 and 3,939,254, and Pat. No. 4,058,667. As used in such apparatus, it is preferable to employ a linear electron beam evaporation source as disclosed in the above-referenced application to provide a long line source of uranium vapor which flows radially outward from the line of vaporization, for subsequent isotopically selective ionization and separation. Such a vapor source is also preferably operated at a high vaporization rate for increased enrichment efficiency.
In order for an atomic laser isotope separation process to be efficiently performed, it has been found that it is generally necessary and desirable for the vapor density along any line parallel to the linear vapor source to be uniform. Since the vaporization rate varies exponentially with the surface temperature of the melt, this translates to a requirement that the current per unit length emitted by the electron beam gun be constant at all points along the length of the gun.
Experience has shown, however, that it is extremely difficult to maintain a uniform emitted electron beam current density over the length of the filament. One factor contributing to such difficulty is that portions of the surface of the filament may become coated with uranium (or other melt material), which results in increased electron emission from the affected region by virtue of a local decrease in the effective work function of the surface. This situation is aggravated further to the extent that the increased local current density results in a higher surface temperature for the adjacent portion of the melt, with consequent increased vaporization rates and even greater coating of the filament. Consequently, an unstable situation may result, wherein it is impossible to maintain uniformity of the emitted current per unit length along the electron gun's lengthwise dimension, even though the total current for the filament may be controlled by means of feedback circuitry. In one particular case, a condition of nonuniformity has been observed, so egregious that a portion of the electron beam bored a hole right through the melt, into the bottom of the crucible.